Are locks unnecessary in multi-threaded Python code because of the GIL?

Question

If you are relying on an implementation of Python that has a Global Interpreter Lock (i.e. CPython) and writing multithreaded code, do you really need locks at all?

Answer 1

This post describes the GIL at a fairly high-level:

• https://web.archive.org/web/20080516010343/http://www.pyzine.com/Issue001/Section_Articles/article_ThreadingGlobalInterpreter.html

Of particular interest are these quotes:

Every ten instructions (this default can be changed), the core releases the GIL for the current thread. At that point, the OS chooses a thread from all the threads competing for the lock (possibly choosing the same thread that just released the GIL – you don't have any control over which thread gets chosen); that thread acquires the GIL and then runs for another ten bytecodes.

and

Note carefully that the GIL only restricts pure Python code. Extensions (external Python libraries usually written in C) can be written that release the lock, which then allows the Python interpreter to run separately from the extension until the extension reacquires the lock.

It sounds like the GIL just provides fewer possible instances for a context switch, and makes multi-core/processor systems behave as a single core, with respect to each python interpreter instance, so yes, you still need to use synchronization mechanisms.